(A) I’ll write a letter when I will reach Lahore.
(B) I’ll write a letter when I reach Lahore.
(C) I’ll write a letter as I reach Lahore.
(D) I’ll write a letter when I shall reach Lahore.
Explanation:
When we refer to the future in an adverbial clause of time, we normally use the simple present tense ( not will or shall).
